Fix compilation on Windows when using standalone clang

[deimonn]

Trying to compile gc.c using Clang 18.1.8 on Windows (the standalone version from the installer found in LLVM's releases page, not the MSYS2 version; that works fine) generates an "undeclared library function _AddressOfReturnAddress" error when expanding the __builtin_frame_address macro.

Clang supports the __builtin_frame_address built-in on Windows even in the MSVC environment, so the macro doesn't even need to exist when compiling with it.

This PR fixes the issue with a little check to omit the macro when __clang__ is defined.

[deimonn]

I just realized (that's what I get for not looking beforehand) that #62 kind of addresses this, although GrimMaple's approach is to bring in intrin.h instead of using the __builtin_frame_address already provided by Clang.

I prefer this approach for consistency, but it really is up to preference.